An Energy-Efficient Solar-Heated Greenhouse Produces Cool-Season Vegetables all Winter Long

arious strategies are commonly used to extend the spring and fall growing seasons by a few weeks, but those bleak mid-winter months leave many of us yearning for anything fresh and crunchy that has not been shipped from thousands of miles away. An energy-efficient, solar-heated greenhouse can provide an inexpensive and successful tool to fill the gap for those of us with winter garden-envy. A variety of cold-hardy species, including vegetables, salad crops, restaurant garnishes, and fresh cut flowers can be successfully and economically cultivated in a properlydesigned solar-heated winter greenhouse with little or no added heat. A successful example of such a greenhouse was erected at the University of Missouri's Southwest Research Center near Mt. Vernon in 1988. This simple inexpensive structure is energy efficient and heated only by the sun. For more than ten winters, it has proven to be an ideal winter sanctuary for growing nearly any cool-season crop imaginable, including salad greens, broccoli and spinach (see Table 1). The Southwest Center's solar-heated

greenhouse is situated on the top of a windy, exposed hill in southwest Missouri, the logic being that if it will work at this challenging site, it should work almost anywhere in reasonably mild, temperate climates. In keeping with the theme of simplicity these cool-season crops were grown with no supplemental heat or light to determine how they would fare. Crops were usually direct-seeded into the raised beds in early November, with some fast-growing crops such as lettuce being successively seeded throughout the season. Of course, plants raised elsewhere could easily be transplanted into the greenhouse in November providing a substantial jumpstart on the season. Usually by the first of April the greenhouse becomes too warm and is shut down for the summer.

Design
Over the last 25 years, many different greenhouse designs and technologies intended to maintain a favorable winter greenhouse climate at low cost have been studied. Many of the most successful passively-heated greenhouses take advantage of the thermal storage capabilities of either

insulation. provide easy access. inches with the other being Leafy greens Vegetables Flowers The north. and the northallow for a small entrance Cilantro Broccoli Linaria (Toadflax) sloped roof portion are 6area. An insulated metal door was installed rotted after about eight years of use.500 by C. fertile mixture of Cress Carrot fiberglass batt insulation. low profile design. Numerous cultivars of the following cool-season crops even the smallest amount using treated lumber in this were successfully grown over several winters in the Southwest of heat from inside the type of environment. The south side features a sloping 45° face Figure 1. 4” The Southwest Center's greenhouse relies on its efficient. This short south wall was originally built of untreated lumber. efficient greenhouse was built of low-cost and recycled materials for approximately $1. greenhouse. separated by a narrow walkway to accumulation of water between the two layers of plastic. plastic. Although we have not tried this. All interior absorb more solar energy and Purslane surfaces are painted white warm up better than low. In addition to creating Escarole Onion faced with plywood and a deep well-drained. One Center’s solar-heated greenhouse with no supplemental heat. two layers of plastic. but do not want to remove we would not recommend Table 1. A small electric blower (60 cubic feet per into the greenhouse at night—a critical component of this minute capacity) is used to pump outside air between the very successful system. such as in water-filled plastic sleeves placed on or below the soil surface. A row of 20 black 55-gallon plastic barrels. therefore it was on the east side. Metal barrels would be an option. humid constructed directly on the soil floor of the greenhouse with greenhouse air would lead to condensation and an untreated pine lumber. which to protect the wood. we The growing area consists of two raised garden beds suspect that inflating the layers with warm. Stored water can be integrated into a greenhouse design in various ways. inflating it to provide rigidity and but they may be prone to rust problems.100 gallons of water to capture and finishing boards at the edge of the structure and nailing store solar heat in the daytime and slowly release it back them in place. we simply replaced every few years. or held in large containers placed in 6’ x various positions inside the structure. This simple. These untreated boards will need to be Furthermore. The building measures 12x24 feet and is 11 feet high at its peak. resting on a 34-inch high vertical wall (Figure 1).rock masses or large volumes of water. For stability and solid footing the barrels sit on a surfaces are glazed with a double layer of 6-mil greenhouse narrow concrete pad that runs the length of the building. that is 12x24 feet. to Chard Bok choi Johnny jump-up west walls. and Arugula Beet Calendula slightly shorter at 19 feet. Any containers insulation. total growing area is Collards Broccoli raab Pansy inch stud-wall about 123 square feet. these Mizuna Pea protective sheet metal raised beds of dark soil may Mustard siding. flat Radicchio Spinach to be highly reflective and beds. fertile Lettuce Pak choi the exterior sheathed with growing medium. be dark-colored or painted black to pumping air from inside the greenhouse would provide maximize absorption of solar heat. A common question that is asked is whether chosen should of course. line the north soil or plants being grown. better insulation. A Cornsalad (Mache) Cabbage construction with R19 wholesome. east. and it has bed measures 22x3 feet by 10 worked just fine as it is. topsoil and compost fills the Dandelion Kale Endive Leek These interior walls were beds. and a large volume of water to moderate interior temperatures. which was installed simply by rolling the edges over The barrels provide 1. and are much easier to
. replaced with treated lumber that is not in contact with the stacked two-high and filled with plain water. Both of these south-facing wall. Daniel Wooley.

the water bitterly cold nights if desired. Near the peak of the roof in the While outside temperatures are east wall we installed a logically the most influential in thermostatically-controlled ventilation determining minimum greenhouse fan with 1. greenhouse temperature can easily rise minimum greenhouse temperature. in heating the greenhouse at night was estimated by measuring the daily change in water temperature. The stored water is. While winters. or for example. unheated structure. while maximum water temperatures ranged Ventilation from 48-91°F. The minimum water. For more details warm by absorbing heat from the sun the fan can pull the hot air across the top during the day and releasing it into the on these statistical results. temperature changed an average of 8. For more than ten and released back into the greenhouse each night. in the barrels was statistically depending on the crops being grown. The power of the sun is very Our data revealed that the most evident on clear winter days when the important factor horticulturally.130 pounds. and out of the structure without Thomas and Crawford (2001). An irrigation timer could be used but our experience has been that it is better to monitor the water needs of the plants and irrigate when needed. Initially a floor-level air intake water temperature was also found to vent was installed but the cold be statistically significant in ventilation air entering the greenhouse influencing greenhouse soil proved detrimental to plants near the temperatures.0°F and 6. quite acceptable for most cool-season crops. if desired. Irrigation water is provided by a freeze-proof hydrant installed within the structure and weep-wall hoses that run the length of each bed and back. On the coldest night ever recorded while the greenhouse was in operation. The impact of the stored water from blowing open. ventilation temperatures and minimum water also helps manage humidity and CO2 temperature inside the structure. One British thermal unit (BTU) is the quantity of heat both were constructed with two louvers. Temperature respectively. a bitterly cold –13°F in 1996 did not even phase the thriving plants inside. by placing large pots on top of the water barrels. Minimum daily water temperatures in the barrels fluctuated between 43 and 79°F.100 gallons of water in the greenhouse is the fan is positioned so that the wind cannot blow it open. saving the timer for use when you are away.040 (February) and The success and usefulness of this greenhouse design 62. whereas the exterior louver on the vent is electrically driven approximately 9.
. Detailed temperature data have been collected during several of these winters to document what is really going on inside of the greenhouse. The interior growing space could certainly be increased. To prevent BTU Calculations cold air from seeping in through the fan and vent openings. We prefer this type of drip hose because it distributes water evenly throughout the beds and minimizes water contact with the foliage. nearly any cool-season crop imaginable has been not a tremendous amount of heat. The exterior louver on required to raise one pound of water 1°F. one set on the exterior wall and one on the interior. as the interior temperature held steady at 33°F. and therefore 9. a critical component of this installed high on the west wall so that greenhouse system. growing additional plants in other elevated structures such as suspended guttering. it is useful and capacity.084 (March) Btu's were captured by the water each day continue to be proven year after year. recorded minimum nightly soil temperatures fluctuated between 40 and 56°F. to 110°F or more.successfully grown and harvested within this simple. the greenhouse never dropped below the freezing point. The thermostat is set to intriguing to note that the water held initiate ventilation at 80-85°F. The present intake vent is Water-filled barrels (on right) keep plants therefore. making ventilation a was significantly influenced by necessity. intake vent.800 cubic feet per minute temperature. During several winters. In addition to moderating maximum and minimum outside greenhouse temperatures. In and could be manually covered with a heavy blanket on February and March 1998. levels. for example. Therefore an average of 73. significant in determining the internal in order to help heat the stored interior greenhouse climate. refer to greenhouse at night. Even during one of the coldest winters on record in southwest Missouri (1989-90). it was consistently maintain and water than individually potted plants.8°F daily. The total weight of the 1. again to prevent it temperature drops at night.130 Btu's are released into the greenhouse for each degree F the water open and closed in concert with the fan. impacting the crops inside. These two openings are probably the weakest link in the integrity of the building insulation-wise.

For example.com
. efficient.
modifications can be made to suit each and every person's particular interest. This fortuitous winter greenhouse tomatoes. needs. Fungal root rots caused structure lengthwise rather than in by very wet. An advantage to using this greenhouse is empty during the material inside such a structure is that summer (we usually cover the beds there is no wind that would require with cardboard to prevent weeds) securing the edges. but we would and no pest control measures have encourage the grower to expand the been required. small space that will be diseases to monitor. 3903 W. and inexpensive design works perfectly fixtures. additional artificial lighting would have long for many years to come. 2001. The greenhouse the cool conditions in the greenhouse could be made larger. and by rotating crops. more sophisticated heat and water handling equipment could be Insects and Diseases added. MO 65243. solar-heated greenhouse in southwest Missouri. the beauty of this type of study is that once the basic design has been proven. While all of the crops listed well just as it is and will almost certainly never fail in in Table 1 grew satisfactorily without supplemental providing fresh. Clark. lack of insect and disease pressure makes raising crops cucumbers. and electricity. The beds. Vernon. www.adequate to keep the greenhouse temperature at or above freezing. that might cause concern during the cannot give up the insatiable desire for winter growing season. extra degree of protection (which we diseases have not been a concern in the never needed) would be floating row greenhouse with any of the wide covers placed directly over the raised variety of crops we have grown. different glazing materials Insect pests and diseases have could be used instead of the double never posed a problem with any of the layer of plastic. Another foliage dry and watering infrequently tool that could be used to add that but deeply. Of course. this basic design organically in this setting a cinch.
Reprinted from Small Farm Today magazine. Jr. but would 14548 Hwy H probably only be necessary for the serious commercial Mt. MO 65712. 417-466-2148
Conclusion and Options
This unheated greenhouse has proven to be extremely successful at doing exactly what it was designed for— growing cool-season crops in winter with few. we can unabashedly state that this extremely basic. and other heat-loving crops.smallfarmtoday. undoubtedly increased the productivity of most crops while adding a small amount of beneficial heat to the greenhouse For additional information. By keeping impacted by the stored water. the structure could be crops cultivated from November built into an existing structure or sunk through March. For those who simply grow on a cold January day. nutritious. With all the possible modifications and soArtificial lighting has not been used in this greenhouse called improvements that could and will be made to this to determine if sufficient light would be available in winter design. variations and
Literature Cited
Thomas. Insects and mites into a south-facing hill. could certainly add tremendous energy efficiency to a greenhouse where substantial additional heat and light Lighting were used. and situation. and Richard J. to grow crops without the added complexity and expense of simple. Lights could be used on cloudy days and/or Southwest Research Center in the evenings to extend the day length. peppers. Contact Small Farm Today. Performance of an energy-efficient. Ridge Trail Rd. contact the authors at: environment. Crawford. cool soil conditions would width and height to maintain the probably be the most important intimate. crunchy vegetables all winter lighting. University of Missouri-Columbia. grower. etc. a rock mass either remain dormant or are killed by could be added. inputs. the material could which also helps reduce the therefore be very easily installed and proliferation of insects and diseases Ten varieties of heirloom lettuce happily removed. March 2003. Southwest Missouri Agricultural Research and Education Center 2001 Research Report. if any. 1-800-633-2535. bulbs. Andrew L.